1. Field of the Invention
The present invention generally relates to a process and an apparatus for producing a dental filling for restoration of a tooth crown, and particularly to the technique of producing such a dental filling with ease and with high form accuracy.
2. Related Art Statement
As one of known dental restoration techniques of repairing or filling a damage of hard tissue of a tooth crown and thereby restoring the function of the tooth, there is the technique of preparing a cavity in the tooth crown by cutting and grinding the damaged hard tissue and then fitting to the cavity a solid, dental filling (e.g., inlay, onlay, core, crown, bridge) which has been produced to have an external shape or form corresponding to the cavity.
A dental filling for restoration of a tooth crown is formed of metal, porcelain, composite resin, etc. In particular, metal and porcelain are preferable filling materials, because those materials have high strength and high abrasion resistance. Whichever material is selected, an impression (i.e., imprint of a tooth) is taken from a tooth in which a cavity has been prepared, and a model (i.e., replica of the tooth) is formed of, for example, plaster on the impression. This model is used for producing a dental filling to be fitted to the cavity, in a manner suitable for the selected material.
More specifically, in the event that metal is selected as filling material, it is a common practice that a model is used for forming a wax pattern corresponding to a tooth including a cavity; the wax pattern is immersed in a fireproof investment such as plaster; the plaster is heated for removing or evaporating the wax pattern and thereby obtaining a mold; the mold is used for casting the metal; and the cast metal is cut, ground and polished into a final, dental filling. Meanwhile, in the event that porcelain is selected, it is common that a model is used for producing a secondary model; wet porcelain mix is applied to the cavity of the secondary model, and then they are fired; the fired porcelain is cut, ground and polished into a final dental filling.
However, any of the known techniques in which various sorts of materials are used for producing a dental filling, essentially requires taking an impression of a tooth, forming a model on the impression, and producing a pattern or a secondary model using the model. Thus, those techniques are very cumbersome to carry out, need a long time, and cost high.
In addition, since, in the conventional dental filling production techniques, information representing the external shape or form of a tooth having a cavity, is transmitted to a final dental filling via a plurality of intermediates such as a model, pattern, or secondary model, dimensional errors accumulate because of deformation, shrinkage, and/or expansion of the materials of those intermediates. Thus, it has been very difficult to produce a dental filling with desired dimensional accuracy. Accordingly, a skillful technician is needed for the dental filling production. However, recently, dental technicians have been short in number, even posing a social problem.
Furthermore, while attention has been directed to using porcelain as a filling material because of its excellent color tone and gloss, high corrosion resistance, and high mechanical strength, a satisfactory technique of producing a dental filling using porcelain has not been established yet. The reasons for that are as follows: It is very difficult to control firing of porcelain under high temperature and high pressure. Although porcelain shrinks, i.e., reduces in volume, more than several tens percent due to the firing, it is very difficult to take into account, in advance, the amount of shrinkage of the porcelain, unlike industrial products. In addition, dental fillings are required to have different external forms corresponding to individual teeth. Thus, no conventional technique can provide a porcelain dental filling with desired qualities, in particular, mechanical strength, at low cost.